Python list variables not getting modified by if-statement clauses

I am making a game using the Langton's ant algorithm. And I want the list tiles to update the number to 0 in this case... But it doesn't. Why? Notes: - I am using python 2.7

- The direction variable is based on a compass(n, e, w, s)

posx = 4
posy = 4
direction = 'w'

tiles = [[1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1],
     [1,1,1,1,1,1,1,1,1,1]]
def posision(posx, posy, tiles, direction):
    if tiles[posx][posy] == 1:
        tiles[posx][posy] = 0
    if tiles[posx][posy] == 0:
        tiles[posx][posy] = 1

    oldTiles = tiles

    if direction == 'n':
        if oldTiles[posx][posy] == 1:
            posx = posx+1
            return 'w', tiles
        if oldTiles[posx][posy] == 0:
            posx = posx-1
            return 'e', tiles
    if direction == 's':
        if oldTiles[posx][posy] == 0:
            posx = posx+1
            return 'w', tiles
        if oldTiles[posx][posy] == 1:
            posx = posx-1
            return 'e', tiles
    if direction == 'e':
        if oldTiles[posx][posy] == 1:
            posy = posy +1
            return 'n', tiles
        if oldTiles[posx][posy] == 0:
            posy = posy -1
            return 's', tiles
    if direction == 'w':
        if oldTiles[posx][posy] == 0:
            posy = posy +1
            return 'n', tiles
        if oldTiles[posx][posy] == 1:
            posy = posy -1
            return 's', tiles

direction, tiles = posision(posx, posy, tiles, direction)
print(tiles)